1. Field of the Invention
This invention relates to a method for improving the thermal stability of phosphorodithioic acid diesters. More particularly, it relates to a method for stabilizing such materials which involves the addition of a minor amount of a stabilizer.
2. Description of the Prior Art
The preparation of phosphorodithioic acid diesters by the reaction of a phosphorus sulfide with alcohols or phenols is a well known process which can be described in general terms by the following equation where R represents a hydrocarbyl or substituted hydrocarbyl group and phosphorus pentasulfide is employed as the phosphorus sulfide starting material: ##STR1## The phosphorodithioic acid diesters produced by this process are typically accompanied by small amounts of other related materials, such as P(S)(OR).sub.2 (H), P(S)(OR).sub.2 (SR), P(S)(OR).sub.3, P(O)(OR).sub.2 (SH), P(S)(OR)(OH).sub.2, P(S)(OR)(OH)(SH), (RO).sub.2 (S)P-S-P(S)(OR).sub.2, and (RO).sub.2 (S)P-SS-P(S)(OR).sub.2. In addition, small amounts of elemental sulfur are frequently observed as a by-product of the above-mentioned reaction.
The phosphorodithioic acid diesters can be converted to their corresponding metal salts by reaction with basically reacting metal compounds such as oxides, hydroxides or carbonates. These metal salts are known to be useful as antioxidants and corrosion inhibitors. The zinc salts of phosphorodithioic acid diesters are particularly useful as additives for crankcase lubricants for use in internal combustion engines since they are excellent antioxidants and are also highly effective in reducing the wear and corrosion of the lubricated engine components. For example, these zinc salts protect corrosion-susceptible nonferrous metal components of the engine, such as bearings, against corrosive contaminants in the lubricating oil. Such contaminants are primarily acidic products of oil oxidation and acidic products from the combustion of fuel which blow past the piston rings and into the crankcase. It is believed that the zinc salts of the phosphorodithioic acid diesters function as corrosion inhibitors for nonferrous engine parts, such as copper-lead or lead-bronze faced bearings, by reacting chemically with the surface of these parts to form a protective corrosion-resistant film.
Although the zinc salts of phosphorodithioic acid diesters are highly desirable antioxidants and wear and corrosion inhibitors for use in crankcase lubricants, it is also desirable to use no more of these phosphorus-containing compounds than is necessary to provide the required protection against oxidation, wear and corrosion. This, of course, is a partial consequence of the fact that such additives are relatively expensive in comparison with the mineral oils which are typically used as lubricant base oils. More important, however, is the fact that phosphorus is considered to be harmful to the catalytic converters which are currently employed to control the emission of hydrocarbons, carbon monoxide and nitrogen oxides from automotive engines. In view of this, it is desirable to minimize the phosphorus content of crankcase oils for internal combustion engines which are in automotive service since some of the phosphorus from an engine's lubricant ultimately finds its way into the catalytic converter by way of oil leakage past the piston rings and into the combustion chambers of the engine.
Zinc salts of phosphorodithioic acid diesters wherein at least one of the two ester groups is derived from a secondary alcohol are typically more active as antioxidants and as corrosion and wear inhibitors than the corresponding materials wherein the ester groups are derived from primary alcohols. The use of these more active zinc salts is desirable in the preparation of crankcase lubricants since they permit the formulation of lubricant compositions which have a lower phosphorus content than would be possible with the less active materials. Unfortunately, the phosphorodithioic acid diesters which are obtained by the reaction of phosphorus sulfides with secondary alcohols are usually much less stable than the corresponding materials which are derived from primary alcohols. Further, the manufacture of the zinc salts of these secondary alcohol derivatives is frequently difficult because of this lesser stability. As a consequence, there is a need for a method of stabilizing the phosphorodithioic acid diesters which are derived from the reaction of phosphorus sulfides with secondary alcohols.
U.S. Pat. No. 3,361,668, issued to Wiese on Jan. 2, 1968, discloses that when phosphorodithioic acid diesters are prepared by the reaction of an alcohol or phenol with phosphorus pentasulfide, a product of reduced coloration can be obtained either by adding a small amount of an amine to the alcohol or phenol prior to reaction with the phosphorus pentasulfide or by adding a small amount of the amine to the diester subsequent to its formation. This patent does not, however, suggest that the thermal stability of a diester of phosphorodithioic acid could be improved by any means and does not suggest the use of any materials other than amines to improve the properties of such a phosphorodithioic acid derivative.
U.S. Pat. No. 4,083,899, issued to Demarcq on Apr. 11, 1978, discloses that the preparation of phosphorodithioic acid diesters by reaction of phosphorus pentasulfide with alcohols or phenols can be catalyzed by the use of a wide variety of nitrogen-containing compounds in an amount which ranges from 0.01 to 10% by weight based on the alcohol or phenol. Similarly, British patent specification No. 1,228,528 discloses that from 1 ppm to 1% by weight of ammonia can be used to catalyze the reaction of phosphorus sulfides with alcohols and phenols. In addition, U.S. patent application Ser. No. 242,107 by Sabol et al. teaches that the neutralization reaction between a phosphorodithioic acid diester and a basically reacting zinc compound can be catalyzed by ammonia and ammonia-yielding compounds. However, these references fail to suggest that the thermal stability of a diester of phosphorodithioic acid could be improved by any means.